1. Field of the Invention
The present invention relates to a borehole gravity meter system and, more particularly, to such a system which includes electronic control circuitry to, upon initiation, balance or null a gravity sensing device within the gravity meter and thereafter calculate and display a representation of the gravity at a selected depth within the borehole.
2. Background of the Invention
The use of gravity meters to measure the gravity at a location within a wellbore is well known in the art. One type of borehole gravity meter is known as a LaCoste and Romberg gravity meter which will be described in more detail below. Basically, a gravity meter of the LaCoste and Romberg configuration includes a gravity sensing device which is balanced or "nulled" and the adjustments made to null the device are interpreted to provide an indication of the gravity at a position in the wellbore.
The gravity sensing device generally comprises a hinged horizontal beam with a mass connected to one end thereof. The beam is unclamped to allow for its free movement after the gravity meter has been stopped within the borehole. A spring is connected at one end to the beam and at an opposite end to a stationary point in the gravity meter. The spring applies an upward force to the beam and mass and the tension of the spring and thus the position of the beam and mass can be adjusted by the operation of a motorized measuring screw. The mass is free to move between two parallel horizontal plates which are part of a capacitive position indicator (CPI) circuit. The position of the mass is detected by this CPI circuit and its output is sent to a chart recorder on the surface. The operator views the chart recorder and causes the measuring screw to move to adjust the spring tension until the mass is in balance between the downward force of gravity on the beam and mass and the upward force of the spring. This is called nulling the meter, i.e., there is no motion in the beam and mass when a balance has been achieved and the mass is approximately equal distance between the two CPI plates. Once a null has been obtained, a gravity reading can be determined by taking the amount of movement of the measuring screw to achieve the null and performing several simple calculations. The beam is then clamped and the gravity meter can be moved to the next location where the procedure is repeated. This method of obtaining a gravity measurement is laborious and fatiguing to the operator because of the attention and skill needed to watch the chart recorder and adjust the spring tension to achieve a null.
Certain borehole gravity meter systems use a microprocessor to continuously level the gravity meter. However, because of factors such as noise and mechanical hysteresis, the nulling and reading of the measuring screw position still has to be done manually in this type of gravity meter. The result of mechanical hysteresis is that a null achieved with the measuring screw rotated in one direction will be different from a null achieved with the measuring screw rotated in the opposite direction. Therefore, the operator needs to be careful in rotating the measuring screw in only one direction to achieve a null. Noise, such as hole noise, earthquakes or small tool movement, has to be filtered out or disregarded as to its cyclic content before any corrections to the measuring screw position can be made or an overshoot of the correct reading will occur, i.e., the operator must judge the cyclic motion of the beam on the chart recorder caused by the noise before making a correction to the measuring screw position. This ability to evaluate the cyclic motion requires capabilities of pattern recognition which if done electronically, presently requires large and powerful computing systems.
To overcome some of these problems, electrostatic forces can be applied to the mass to move it to a desired position, as illustrated in "Measurements in the Earth Mode Frequency Range by an Electrostatic Sensing Gravimeter" by Barry Block and R. D. Moore, Journal of Geophysics, Research (1971) 18, 4361-4375 (1966), which is incorporated by reference. In the system of Block and Moore, the measuring screw is rotated to move the mass to a null position while a voltage is applied to the CPI plates. The movement of the mass between the plates causes changes in the voltage readings which are monitored to determine the mass position. The mass' range of movement caused by the electrostatic forcing of the CPI plates can be equivalent to several milligals so the measuring screw is seldom used in systems as described in the Block and Moore paper (supra); however, in the borehole, the gravity to be measured during a survey varies over a range on the order of several hundred milligals and as such, the measuring screw will be used regularly.
A significant problem with the operation of all of these gravity meters is the attentiveness and skill required of the operator to read the chart recorder and adjust the measuring screw position to null the position. If a reading is taken when the meter is not leveled or nulled, obviously the gravity measurement will be inaccurate. In the field, the quality of the operators varies so the quality of the gravity measurements can vary. These types of gravity measurements are time consuming and an operator can attempt to obtain an accurate gravity measurement by watching a chart recorder after having been on the job for over 24 hours straight. After a null has been achieved, the position of the measuring screw is taken and the operator transcribes the measuring screw position data into a written log and also perform some hand calculations to obtain a gravity measurement. This need for operator transcription can also be a source of errors in the gravity surveys.
The inventors hereof know of no borehole gravity meter system which nulls the meter upon operator initiation and internally calculates the gravity reading without the need for human reading and transcription.